Don changes the can to allow the use of manifold vacuum at idle. I now run this distributor which is connected to manifold vacuum and my N idle is rock steady.
Note: Once I know what the lowest rated vacuum rating is per FBO, then I will tune my in D idle to make sure that I have at least that amount so that my in D rpms (with brake applied) does not fluctuate.

If a distributor has a higher rated can and you try to hook it up to a low manifold vacuum, than I can understand why the idle would wonder. But if the distributor is setup to run manifold vacuum (lower rated can), and you have enough vacuum at idle (or D for an auto), then there should not be any issues with the rpms fluctuating.

I found this description on the net:

""Vacuum advance calibrations are different between stock engines and modified engines, especially if you have a lot of cam and have relatively low manifold vacuum at idle. Most stock vacuum advance cans aren’t fully-deployed until they see about 15” Hg. Manifold vacuum, so those cans don’t work very well on a modified engine; with less than 15” Hg. at a rough idle, the stock can will “dither” in and out in response to the rapidly-changing manifold vacuum, constantly varying the amount of vacuum advance, which creates an unstable idle. Modified engines with more cam that generate less than 15” Hg. of vacuum at idle need a vacuum advance can that’s fully-deployed at least 1”, preferably 2” of vacuum less than idle vacuum level so idle advance is solid and stable; the Echlin #VC-1810 advance can (about $10 at NAPA) provides the same amount of advance as the stock can (15 degrees), but is fully-deployed at only 8” of vacuum, so there is no variation in idle timing even with a stout cam.""